IEEE 802.11s describes an amendment for Mesh Wireless Local Area Network (WLAN). The proposal foresees different optional enhancements to enhance the current Medium Access Control (MAC). IEEE 802.11e describes the mandatory MAC of IEEE 802.11s. It is called Enhanced Distributed Channel Access (EDCA). EDCA is designed for single-hop wireless networks. It provides four different access categories for prioritization. However, EDCA cannot exploit multiple frequency channels. Thus, with a single radio frequency transceiver only a single frequency channel may be used throughout the entire Mesh WLAN.
The Common Channel Framework (CCF) is an optional method proposed in IEEE 802.11s. It provides means for switching from one frequency channel to another frequency channels using a single transceiver. Thus, separate frequency channels can be used by Mesh WLAN devices.
FIG. 1 illustrates the CCF operation. When switching from one frequency channel to another frequency channel, a station that applies CCF solely relies on physical Carrier Sensing (P-CS) to detect if this another frequency channel is idle or not. While the transceiver of this station is tuned to the common frequency channel f0, other stations may exchange data frames on frequency channels other than f0. Using Request to Change (RTX) and Clear to Change (CTX) frames, CCF enabled stations agree to exchange data frames on frequency channels other than f0. The request to change and the clear to change contain a duration d, which defines the period necessary to transmit the actual data frames and the returning acknowledgement ACK. SIFS (Short Interframe Space) represents the shortest duration that is used for transceiver turnaround whenever a transmitter changes to receive mode and vice versa.
Let us assume that a pair of CCF enabled stations have decided to switch to frequency channel fn to exchange data frames. As this pair of stations cannot sense the frequency channel fn before switching, the pair of CCF enabled stations cannot detect if there is ongoing transmission on the frequency channel fn. Thus, they may detect a busy channel after switching, namely a frequency channel already used by CCF unaware stations, and therefore may not be able to exchange data frames.